Lithium intercalation compounds such as Li.sub.1+X Mn.sub.2-X O.sub.4+Y, LiCoO.sub.2, and LiNiO.sub.2 have been used in positive electrodes for 4 V and higher than 4 V secondary lithium and lithium-ion batteries. These batteries typically include (1) a positive electrode including a lithium intercalation compound, (2) a negative electrode formed of lithium metal, a lithium alloy or a carbon compound, (3) an electrolyte based on an inorganic lithium salt dissolved in organic solvents, and (4) an appropriate separator.
One of the main drawbacks of 4V or higher secondary lithium and lithium-ion batteries is electrolyte decomposition during the charging process or during the shelf life of the battery in its charged state. The negative effects of this decomposition are considerably accelerated at elevated temperatures. Accordingly, to decrease electrolyte decomposition in conventional cells, low voltage limits are strictly used during the cell charge process.
Another drawback of electrolyte decomposition is that the decomposition products either tend to polymerize or tend to initiate electrolyte polymerization, particularly in solvents containing cyclic esters. This polymerization can block the compartment between the electrodes and cause failure of the cell.
Furthermore, when manganese-rich and cobalt-rich lithiated metal oxides are used as positive electrode materials, manganese and cobalt dissolution can occur in the cell. This dissolution is observed in the electrolyte and results in a reduction in the capacity and cycleability of the cell. In particular, the negative effect of manganese dissolution is more pronounced because it is believed that the dissolved manganese catalyzes electrolyte polymerization and/or decomposition.